<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }

    .box {
      display: flex;
      align-items: center;
      /* 拉伸， 默认值（去掉子级的高度才能看成效果）*/
      align-items: stretch;
      height: 300px;
      margin: auto;
      border: 1px solid #000;
    }

    .box div {
      /* width: 100px;
      height: 100px; */
      background-color: skyblue;
    }

    .box div:nth-child(2) {
      align-self: center;
    }
  </style>
</head>
<body>
  <div class="box">
    <div>ssssss1</div>
    <div>2</div>
    <div>3</div>
  </div>
</body>
</html>